Alkaline storage batteries, such as nickel-metal hydride storage batteries, nickel-cadmium storage batteries, and nickel-zinc storage batteries, include a positive electrode, a negative electrode, a separator intervening between the positive electrode and the negative electrode, and an alkaline electrolyte. The positive electrode is a nickel electrode containing nickel hydroxide, and the negative electrode is a hydrogen storage alloy electrode, a cadmium electrode, a zinc electrode, or the like. Also, the separator is commonly made of a polyolefin non-woven fabric, and the alkaline electrolyte is an aqueous potassium hydroxide solution or the like (Power Sources 12, Research and Development in Non-mechanical Electrical Power Sources, 1988, pages 393-410).
The above-described conventional alkaline storage batteries have the following problems.
In the case of nickel-metal hydride storage batteries, when they are repeatedly charged and discharged at high temperatures, the hydrogen storage alloy contained in their negative electrode reacts with an electrolyte and thus corrodes to produce an oxide or hydroxide. Since this reaction consumes water contained in the electrolyte, the occurrence of corrosion of the alloy decreases the electrolyte retained in the separator, increases the internal resistance of the battery, and shortens the life of the battery. Also, due to the corrosion, constituent elements of the hydrogen storage alloy leach into the electrolyte and pass through the separator to reach the positive electrode. This promotes the self-discharge of the battery, thereby resulting in a degradation in capacity retention performance. Therefore, in order to suppress the corrosion of the hydrogen storage alloy, active research has conventionally been conducted on the composition of the hydrogen storage alloy, the surface treatment of the alloy and the like.
In the case of nickel-zinc storage batteries, zinc and/or zinc oxide contained in the zinc electrode are highly soluble in an alkaline electrolyte. Therefore, such batteries are susceptible to the formation of dendrites with charge and discharge cycles. Dendrites are a cause of internal short-circuits.
In view of the problems as described above, improvement of the separator has been attempted.
For example, there has been a report that improving the hydrophilicity of the separator prevents the internal resistance of a battery from rising even when the amount of the electrolyte is decreased in the battery. However, since an additional process is necessary for improving the hydrophilicity of the separator, there is a problem of increasing the manufacturing costs of batteries.
Also, Japanese Laid-Open Patent Publication No. Hei 5-258767 proposes the inclusion of a water-absorbing polymer in an electrolyte, in order to obtain an alkaline storage battery with little self-discharge during storage. This method, however, has a drawback in that the water-absorbing polymer is unevenly distributed between the positive electrode and the negative electrode. Thus, the battery reaction proceeds unevenly, so that there is little expectation of cycle life improvement.
Meanwhile, U.S. Pat. No. 5,541,019 proposes the use of a polymer electrolyte in nickel-metal hydride storage batteries, in order to prevent leakage of the electrolyte. However, if sealed nickel-metal hydride storage batteries are overcharged, electrolysis of water occurs, thereby resulting in gas generation. In this case, there is a problem in that the inner pressure of the battery tends to rise, because the polymer electrolyte has low gas permeability.
In view of the above problems, Japanese Laid-Open Patent Publication No. 2002-33093 proposes the use of a separator layer including a water-absorbing polymer, a water repellent and an alkaline aqueous solution in nickel-metal hydride storage batteries, in order to improve both hydrophilicity and gas permeability.
Further, International Publication WO 02/23663 proposes the use of a separator layer including non-woven fabric and a water-absorbing polymer in nickel-zinc storage batteries, in order to enhance the ability to retain an alkaline aqueous solution while limiting the movement of zinc leached in an electrolyte to suppress the formation of dendrites.